Yamaha MX-1000 protection light, outputs all good, what's next?

[skiracer]

I have a Yamaha MX-1000 that I have been rebuilding/re-capping and have an issue very similar to this one. The unit powers up but immediately goes into protect mode. I am getting zero idling current voltage across test resistors R578 and 579, and RE, LE, E, RO, and LO are all zero volts. Going to the amp board interfaces I get (in Volts dc):
Rch / Lch
TP ; 0 / -91.1
OUT ; 0 / -91.1
IN ; 0 / -91.1
PR ; 0 / 0.3
E ; 0 / 0
+HB; 93.8 / 93.8
-HB ; -93.8 / -93.8
+LB ; 40.98 / 40.98
-LB; -40.98 / -40.98

During short duration testing R347 is getting very hot, likely due to the 91V on the Lch out. Clearly something with the left channel is amiss. I tested all the output transistors when I had the amp boards out and they all tested good.

Any ideas for further testing or troubleshooting? There are no obvious burned resistors or capacitors anywhere. I'm not an EE but have some circuit experience.
Thanks!

 

[rottalpha]

I have a Yamaha MX-1000 that I have been rebuilding/re-capping and have an issue very similar to this one. The unit powers up but immediately goes into protect mode. I am getting zero idling current voltage across test resistors R578 and 579, and RE, LE, E, RO, and LO are all zero volts. Going to the amp board interfaces I get (in Volts dc):
Rch / Lch
TP ; 0 / -91.1
OUT ; 0 / -91.1
IN ; 0 / -91.1
PR ; 0 / 0.3
E ; 0 / 0
+HB; 93.8 / 93.8
-HB ; -93.8 / -93.8
+LB ; 40.98 / 40.98
-LB; -40.98 / -40.98

During short duration testing R347 is getting very hot, likely due to the 91V on the Lch out. Clearly something with the left channel is amiss. I tested all the output transistors when I had the amp boards out and they all tested good.

Any ideas for further testing or troubleshooting? There are no obvious burned resistors or capacitors anywhere. I'm not an EE but have some circuit experience.
Thanks!

zombie thread.

go to the Yamaha forum and post your question there.

as far as suggestions:
disconnect the troubled channel and test the rest of the amp. Heat the pins not the pads when you use your solder iron. The copper tracks / pads on these boards can start to look like this very fast if you are not careful, don't have the skill-set or proper equipment:

 

[mbz]

Rch / Lch
TP ; 0 / -91.1
OUT ; 0 / -91.1
IN ; 0 / -91.1

Don't know the amp but -91.1V at the TP etc... probably indicates failed output transistor.
Start with diode testing outputs.

As above, create a new thread in the yamaha sub forum, guys there have a deep understanding of the amp.

 

[skiracer]

zombie thread.

go to the Yamaha forum and post your question there.

as far as suggestions:
disconnect the troubled channel and test the rest of the amp. Heat the pins not the pads when you use your solder iron. The copper tracks / pads on these boards can start to look like this very fast if you are not careful, don't have the skill-set or proper equipment:
View attachment 3709026

Thanks for the reply, have created a new thread in the Yamaha section. Fortunately the pads have survived my removal of the amp beard (twice now- I'm very careful).

 

[skiracer]

Don't know the amp but -91.1V at the TP etc... probably indicates failed output transistor.
Start with diode testing outputs.

As above, create a new thread in the yamaha sub forum, guys there have a deep understanding of the amp.

Thanks for the reply, I have diode tested all the output transistors and they test soog, no shorted circuits. I have created a new thread in the Yamaha forum.